Fox, on a "basic" level, is pretty flow-charty. Projectile spammers complicate what I'm about to say a bit, but to handle projectile campers, you can always just laser camp them and force them to come to you. Fox is the fastest character in the game and can more or less outrun anybody, at least long enough to make them chase you, which their character may not be equipped to do.
Anyway, on to the flowchart:
1. Dashdance. Fox has the best dash in the game; it's super quick, and just long enough to apply pressure and weave in and out of the opponent's "effective range." Their effective range is the reach of their longest-reaching move that they can use more or less isntantly. For example, watch out for Dedede's ftilit (not that you'll fight many Dedede's but it's a good example). Do not worry about the second swing of Link's F Smash or the later hits of Roy's DED, though, as they take time to come out.
Watch for your opponent to throw out a move that is punishable, then proceed to step 2:
2. Punish. When your opponent leaves themself open, punish with whatever is quick enough to, ie. grab or upsmash. Fox generally combos up, so uthrow into either uthrow, utilt, or usmash, into more of them, into uairs. You can also use Fox's go-to combo, which is nair->shine-wavedash-uthrow or usmash. If they fall over, you can jab them to force them to stand up, then grab. If they fall over and slide too far, just wavedash out of your shine (basically always), and start dashdancing to maintain pressure.
3. Finish. Fox can kill off the top with usmash and uair, edgeguard like a boss with bair, and gimp with shinespikes (especially effective on characters with crappy and/or predictable recoveries, and those with bad offstage games. See Falco, Falcon, even other Foxes.).
Obviously there's a lot more to Fox than this (like multishines), and if it were that easy to just dashdance until your opponent gives you an opening, we'd be playing in 20XX right now. But those are the basics and they should help you get started.
